Without much difficulty you can set up a series of looped slides at the beginning of your slideshow which will advance automatically until you are ready to start the main show. The looped slides could have surprising statistics, facts about the presentations content, trivia questions with a prize given at the end, review questions from the previous class, or advertising space for sponsors.
Depending on your version of PowerPoint the specific menus and steps may vary slightly. The first step is to set up your slideshow as you normally would and then insert the slides you want to repeat at the beginning. Hide all but the slides you want to loop. Using the Set Up Show menu, set the show to “Loop continuously until ‘Esc’ “ and under “Advance slides” select “Using timings, if present”. Exit that menu and in the main slide view, select the slides that will be on loop. Use slide transitions to have them automatically advance after a defined period of time. If you are only displaying a funny image or something similar, 5 seconds is a good amount of time to have the slide on screen. For displaying questions try a longer time like 10 or 20 seconds. When you run the slideshow now it will automatically advance through all of the unhidden slides and loop back to the start of those same slides. When you are ready to begin the main presentation you can right click and select the slide you want to start with.
Bonus tutorial:
For style points, you can create a button that allows you to start the main presentation with a single click rather than pulling up the menu. Start by creating a shape that covers the entire slide and then make it 100% transparent. Right click on it, select action settings and select “Hyperlink to:”. From the drop down, select the first slide of your main presentation, click ok. You just created a completely invisible button to exit the looped slides and take you to your first slide. Copy that button on to each of the looped slides. Now no matter where you are in that loop when you are ready to start class all you have to do is click anywhere in the screen and like magic you are at your main presentation.